Paratime History – And The Paratime Police! (Multiverse)

In the late 1940s and 50s, writers such as H. Beam Piper, Sam Merwin, Jr. and Andre Norton wrote stories set in a multiverse in which all alternate histories are co-existent and travel between them occurs via a technology involving portals and/or paratime transporter machinery.
These authors established the convention of a secret paratime trading empire that exploits and/or protects worlds lacking the paratime technology via a network of secret agents (Piper called them the “paratime police”).